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Old 8th June 2006, 08:32 AM   #1
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Default mosfet dissipation

what`s the safe % of mosfet dissipation?
If max dissipation of a mosfet in datasheet is 150W, could i use it on 120W rms (20V rms x 6A rms).


I`m asking this becouse I know that datasheet numbers of dissipation are bit of optimistic...
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Old 8th June 2006, 08:58 AM   #2
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You need to work back from the wattag rating using the temperature derating curves. Also keep in mind SOA. As a rule of thumb with good thermally efficient insulators (i.e. kapton) and nice heatsink I work to 3/4 rated power.
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Old 8th June 2006, 09:05 AM   #3
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Mosfets are no different from BJTs when it comes to dissipation.
At 50% of the SOA a 150 watt device is only good for 75 watts.

The output devices of an output stage capable of 150 watts in 4 Ohm should therefore have a total power figure of 300 watts minimum.

20V/6A= 3.33 Ohms. Continuous power in 8 Ohms should be close to 75 watts, two 150 watt devices would be sufficient.
But in the words of Nelson Pass: heatsinks are the most important parts of amplifiers.
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Old 8th June 2006, 12:55 PM   #4
suzyj is offline suzyj  Australia-Aboriginal
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It's not as simple as giving some percentage value. You need to do a proper SOA analysis to make an informed judgement.

As an example, here's the quick and dirty thermal analysis I did for my amplifier. The power supply is +/-56V, output maximum power is 100W, and it uses two Hitachi 2SK1058 and 2SJ162 pairs, with thermal transfer pads to a Conrad 300mmx75mmx48mm heatsink:

Rth (transistor) = 1.25 deg/watt
mounting pad = 0.2 deg/watt
Rth (heatsink) = 0.37 deg/watt
Max Temp (transistor) = 150 deg
Dissipation at 50W out = 76.8W (from simulation - 1/2 power point is generally worst case, but it's a good idea to do a few simulations to find the worst case)
Ambient = 25 deg

Temp (transistor) = Ambient + dissipation / thermal resistance

Temp (transistor) = 25 + 76.8 / (((1.25+0.2)/4)+0.37)

Temp (transistor) = 130 degrees.

My design works reasonably comfortably with two pairs of output transistors and a fairly hefty heatsink.

Of course, you should also check that you won't exceed current and voltage ratings for your devices, as well.

Hope that's been of some help.

Cheers,

Suzy
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Old 8th June 2006, 01:20 PM   #5
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Miss Jackson,

your quick calculation is a bit off.
At 76.8 watts dissipation divided over 2 pairs of your Hitachis, the die temperature is 81 C. (with the assumed data you posted)
At a transistor temperature of 105 C above ambient dissipation is 143 watts.

Max dissipation will be a lot higher if you hook your 100 watt in 8 to a 4 Ohm loudspeaker.
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Old 8th June 2006, 01:25 PM   #6
suzyj is offline suzyj  Australia-Aboriginal
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No, the calculation's good, it's just my description that's lacking. The 76W is across all the MOSFETs.



Cheers,

Suzy
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Old 8th June 2006, 04:22 PM   #7
Tim__x is offline Tim__x  Canada
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No, Jacco's right. Using the correct equation,

76.8 * (((1.25+0.2)/4)+0.37) + Ambient

and not

76.8 / (((1.25+0.2)/4)+0.37) + Ambient

There is a 56.256 degree rise, add 25 degrees ambient, and you get Jacco's 81 degrees.
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Old 8th June 2006, 06:51 PM   #8
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Default Re: mosfet dissipation

Quote:
Originally posted by bogdan_borko
what`s the safe % of mosfet dissipation?
If max dissipation of a mosfet in datasheet is 150W, could i use it on 120W rms (20V rms x 6A rms).

I`m asking this becouse I know that datasheet numbers of dissipation are bit of optimistic...
With a heat sink at 55 deg C, I run them continuously at 25 watts
or less, and they don't break. Run them at 50 watts under the
same conditions, and you will lose a small percentage.
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Old 8th June 2006, 09:28 PM   #9
suzyj is offline suzyj  Australia-Aboriginal
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Quote:
Originally posted by Tim__x
No, Jacco's right. Using the correct equation,

76.8 * (((1.25+0.2)/4)+0.37) + Ambient

and not

76.8 / (((1.25+0.2)/4)+0.37) + Ambient

There is a 56.256 degree rise, add 25 degrees ambient, and you get Jacco's 81 degrees.
Eeek! Thanks for that. My only excuse was it was really late at night.

Cheers,

Suzy
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Old 8th June 2006, 09:47 PM   #10
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SJ,

i thought you Bike B. feed the fish and hit the hay at 21.15 ?
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